The chronic lack of donor organs for allotransplantation may be solved if transplantation of organs or tissues from animal to man, i.e. xenotransplantation, could be performed. The pig is considered to be the most suitable donor species for human xenotransplantations, but several problems need to be solved before it can be used on a routine basis. The initial immunological barrier is caused by xenoreactive, natural antibodies (XNAb) in humans, apes and Old world monkeys, that are specific for a carbohydrate epitope, Galα1,3Gal (α-Gal), present in other mammals including pigs. The binding of XNAbs to α-Gal epitopes on pig endothelial cells, initiates a series of events, which leads to graft loss within minutes to hours (Galili, U., 1993; Oriol, R. et al., 1993).